You absolutely right, I thought about it.

Maia

Marius Schmidt wrote:

Interestingly, Maxwell's demon pops up here, whoooo... ,

don't do it.

If you change the reaction rate in one direction 1000  times slower

than

in the other direction, then the reaction becomes practically

irreversible. And the system might not be at equilibrium.

Maia

R. M. Garavito wrote:

Vinson,

As Dale and Randy pointed out, you cannot change the &#916G of a reaction

by mutation: enzyme, which is a catalyst, affects only the activation

barrier (&#916E "double-dagger").  You can just make it a better (or

worse) catalyst which would allow the reaction to flow faster (or

slower) towards equilibrium.  Nature solves this problem very

elegantly by taking a readily reversible enzyme, like an epimerase or

isomerase, and coupling it to a much less reversible reaction which

removes product quickly.  Hence, the mass action is only in one

direction.  An example of such an arrangement is the triose phosphate

isomerase (TIM)-glyceraldehyde 3-phosphate dehydrogenase (GAPDH)

reaction pair.  TIM is readily reversible (DHA <=> G3P), but G3P is

rapidly converted to 1,3-diphosphoglycerate by GAPDH.   The oxidation

and phosphorylation reactions of GAPDH now make TIM "work" in one

direction.

Since many epimerases are very optimized enzymes, why not consider

making a fusion with a second enzyme (like a reductase) to make the

system flow in one direction.  Of course, this depends on what you

want to do with the product.

Cheers,

Michael

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On May 18, 2010, at 11:54 AM, Dale Tronrud wrote:

Hi,

 I'm more of a Fourier coefficient kind of guy, but I thought that a

&#916G of zero simply corresponded to an equilibrium constant of one.  You

can certainly have reversible reactions with other equilibrium constants.

In fact I think "irreversible" reactions are simply ones where the

equilibrium constant is so far to one side that, in practice, the

reaction

always goes all the way to product.

 As Randy pointed out the enzyme cannot change the &#916G (or the

equilibrium

constant).  You could drive a reaction out of equilibrium by coupling it

to some other reaction which itself is way out of equilibrium (such as

ATP hydrolysis in the cell) but I don't think that's a simple mutation of

your enzyme.  ;-)

Dale Tronrud

On 05/18/10 00:31, Vinson LIANG wrote:

Dear all,

Sorry for this silly biochemistory question.  Thing is that I have a

reversible epimerase and I want to mutate it into an inreversible one.

However, I have been told that the &#916G of a reversible reaction is zero.

Which direction the reaction goes depends only on the concentration of

the substrate.  So the conclusion is,

A: I can mutate the epimerase into an inreversible one. But it has no

influence on the reaction direction, and hence it has little mean.

B: There is no way to change a reversible epimerase into an

inversible one.

Could somebody please give me some comment on the two conclution?

Thank you all for your time.

Best,

Vinson

Dr.habil. Marius Schmidt

Asst. Professor

University of Wisconsin-Milwaukee

Department of Physics Room 454

1900 E. Kenwood Blvd.

Milwaukee, WI 53211

phone: +1-414-229-4338

email: [log in to unmask]

http://users.physik.tu-muenchen.de/marius/